X-ray intensifying phosphors are characterized by their ability to absorb X-radiation and to luminesce longer wavelength radiation. The most useful X-ray intensifying phosphors are those which absorb X-radiation and emit, through luminescence, visible or ultraviolet radiation. Medical X-ray intensifying screens represent one commercial application of this phenomenon wherein the screen comprises a phosphor which absorbs the X-radiation and emits an ultraviolet or visible pattern corresponding to the anatomical features of the patient. This imaging process is typically referred to as medical radiography.
The safety of the patient is of utmost concern in the field of medical radiography. In the 100 years since the discovery of diagnostic X-radiation, learned scholars have been seeking advances which will allow for a decrease in the amount of X-radiation exposure required to obtain a diagnostic quality image. Major advances in the art have been achieved. Further improvements are still desired to further decrease the potential effects of patient exposure with ionizing X-radiation.
X-ray phosphors are most commonly prepared by heating phosphor precursors to extremely elevated temperatures in the presence of a flux. The chemical reactions occurring during this process are not well characterized and the art of predicting the solid state reaction chemistry is not well advanced. Therefore, even the most skilled artisan is ill equipped to predict the results one will obtain with variations in the processing procedure.
There are two dominant properties of a phosphor. The first is the ability of the phosphor to absorb X-radiation. The absorption is a function of the atomic number of the chemical elements in the phosphor with larger atomic numbers absorbing more efficiently. The atomic numbers are constant for a given phosphor composition and the process in which the phosphor is prepared does not substantially alter the ability of the phosphor to absorb X-radiation at given particle sizes. A second important property of a phosphor is the ability of the phosphor to convert absorbed X-radiation to ultraviolet or visible light. This is commonly referred to as "conversion efficiency" and the conversion efficiency of a given phosphor is dependent on many parameters most of which are not well understood. It is known that certain impurities and flux compositions can alter the conversion efficiency of a phosphor yet there is no known method of predicting a priori how, or if, the flux composition will effect the conversion efficiency.
One exemplary type of phosphor employed in the art is the family of tantalate phosphors represented by:
(a) YNb.sub.x Ta.sub.1-x O.sub.4, where x is 0 to about 0.15; PA1 (b) LuNb.sub.x Ta.sub.1-x O.sub.4, where x is 0 to about 0.2; PA1 (c) Y.sub.1-y Tm.sub.y TaO.sub.4, where y is 0 to about 0.03; PA1 (d) Y.sub.1-y Tb.sub.y TaO.sub.4, where y is about 0.001 to about 0.15; PA1 (e) Lu.sub.1-y Tb.sub.y TaO.sub.4, where y is about 0.001 to about 0.15; (f) Gd.sub.1-y Tb.sub.y TaO.sub.4, where y is about 0.001 to about 0.15; and various combinations of these tantalate phosphors. PA1 (a) YNb.sub.x Ta.sub.1-x O.sub.4, where x is 0 to about 0.15; PA1 (b) LuNb.sub.x Ta.sub.1-x O.sub.4, where x is 0 to about 0.2; PA1 (c) Y.sub.1-y Tm.sub.y TaO.sub.4, where y is 0 to about 0.03; PA1 (d) a solid solution of (a) and (b); PA1 (e) a solid solution of (a) and (c); PA1 (f) Y.sub.1-y Tb.sub.y TaO.sub.4, where y is about 0.001 to about 0.15; PA1 (g) Lu.sub.1-y Tb.sub.y TaO.sub.4, where y is about 0.001 to about 0.15; PA1 (h) Gd.sub.1-y Tb.sub.y TaO.sub.4, where y is about 0.001 to about 0.15; PA1 (i) a solid solution of at least two of (f), (g) and (h); PA1 (j) any of (a) to (i) wherein up to 45 mole percent of the yttrium, lutetium or gadolinium is replaced by lanthanum; PA1 (k) any of (a) to (i) wherein up to 15 mole percent of the yttrium, lutetium or gadolinium is replaced by ytterbium; PA1 (l) any of (a), (b), (c), (d) and (e) wherein up to 15 mole percent of the yttrium or lutetium is replace by gadolinium; and PA1 (m) Y.sub.1-x Bi.sub.x TaO.sub.4 where x is 0.00005 to about 0.1; PA1 (i) intimately mixing stoichiometric quantities of corresponding precursor oxides; PA1 (ii) mixing the resultant mixture from (i) with a flux consisting essentially of 2-95% by weight KCl, 5-98% by weight of at least one lithium salt chosen from a group consisting of LiCl and Li.sub.2 SO.sub.4 and 0-50% by weight of SrCl.sub.2 ; PA1 (iii) heating the flux-containing mixture from (ii) in the range of about 1100.degree. C., to about 1400.degree. C., for at least 3 hours; and PA1 (iv) recovering the phosphor.
Tantalate phosphors exhibit excellent absorption characteristics due to the incorporation of chemical compounds with large atomic numbers. The conversion efficiency of this class of phosphors is suitable for use in medical diagnostic applications. The applicability of the tantalate phosphors to X-ray intensifying screens was first describe by Brixner in U.S. Pat. No. 4,225,623. The flux composition taught by Brixner comprises Li.sub.2 SO.sub.4, pure LiCl or a BaCl.sub.2 /LiCl eutectic.
Improvements in the conversion efficiency of the tantalate phosphors have been provided. An improved flux composition is provided by Zegarski in U.S. Pat. Nos. 5,064,729 and 5,141,673. These phosphors are based on the use of metasilicates to sequester the generation of alkali metal oxide. The conversion efficiency of the phosphor is improved. Based on the teachings of Zegarski a phosphor composition comprising lithium sulfate should also comprise alkali metal metasilicate. Delayed fluorescence is undesirable in the flux compositions set forth by Zegarski.
The advantages of a LiCl flux, either alone or in combination with Li.sub.2 SO.sub.4 is detailed in Reddy, U.S. Pat. Nos. 4,938,890; 4,929,384; 4,929,385; 4,929,386 and 4,935,161. These combinations provide improvements in conversion efficiency as illustrated in the examples. Yet, a flux comprising either LiCl/Li.sub.2 SO.sub.4 or Li.sub.2 SO.sub.4 /K.sub.2 SO.sub.4 exhibits a level of delayed fluorescence which is undesirable in a commercial embodiment.
Nakajima, U.S. Pat. No. 5,120,619, describes the use of a flux comprising a divalent metal and an alkali metal to decrease delayed fluorescence. Nakajima does not provide a skilled artisan with teachings which would lead to a systematic approach for the discovery of other materials which would decrease delayed fluorescence.
Described herein is an unexpected flux composition which provides a tantalate phosphor with excellent conversion efficiency and decreased delayed fluorescence.